The present invention relates to a servo apparatus and method for high speed reproduction in a digital video cassette recorder (VCR), and more particularly, to a servo apparatus and method which enables a digital VCR to read from a video tape a high efficient coded video signal which has been recorded to be adapted for high speed reproduction.
According to the recommendation proposed by the MPEG (Moving Pictures Experts Group) for a high efficient coding, the highest syntactic structure of the encoded video stream is generally the video sequence. The syntactic structure contains one or more subordinate structures such as a group of pictures (GOP), a picture, a slice, a macroblock and a block.
There are an intra-coded (I) picture (hereinafter called an I-picture), a predictive-coded picture (hereinafter called a P-picture), and a bidirectionally predictive-coded picture (hereinafter called B-picture). These types of the pictures become elements constituting a GOP, in which a coding operation is accomplished in sequence of IBBPBBP . . . . Here, an I-picture is a encoded using information only from itself, a P-picture is a picture which is encoded using motion-compensated prediction from a previous I-picture or P-picture, and a B-picture is a picture which is encoded using motion-compensated prediction from a past and/or future I-picture or P-picture.
When it is required that high efficient coded video data be reproduced from a magnetic tape at a higher speed than a nonpal speed according to a GOP structure, the coded video data is divisionally recorded on the video tape in order to enable an accurate reproduction operation. Such a recording method will be described referring to FIGS. 1 through 3.
FIG. 1 is a conceptual diagram for explaining a recording method for high speed reproduction of high efficient coded video data in a digital VCR. Since an I-picture is a picture encoded by using information only from itself among pictures within one GOP, the encoded video data of the I-picture can be decoded without referring to other picture or pictures. Based on this point of view, the I-picture within each GOP is recorded in the same manner as being distributed at position of tracks indicated as black rectangular boxes on a video tape in FIG. 1, in order to be adapted for high speed reproduction required. The black rectangles shown in FIG. 1 represent portions where the magnetic head always passes during the required high speed reproduction.
According to the MPEG recommendation, an orthogonal transformation coding such as a discrete cosine transform (DCT) is performed for every macroblock having an array of 8.times.8 pixels. Transform coefficients (or frequency coefficients) within one macroblock in a frequency space have DC levels at the upper-left end position. The transform coefficients have the larger AC levels at the closer lower-right end position. The transform coefficients are hierarchically classified according to their magnitudes. As a result, the video data of the I-picture can be disposed on the video tape so as to be adapted for a digital VCR requiring a multiple high speed such as four-, eight- and sixteen-multiple speed reproduction. This is because human's visual characteristic has a lower sensitivity (resolution) with respect to a fast-moving image than that with respect to a slow-moving image. Accordingly, if the transform coefficients of a macroblock are recorded at tape positions on head tracks corresponding to angular speeds for high speed reproduction, a digital VCR can accomplish high speed reproduction adapted for each of the speeds. This is because it is possible to regenerate an effective image for the high speed reproduction by using only the transform coefficients located at the tape positions on the head tracks at the time of the high speed reproduction.
FIG. 2 shows an example of transform coefficients classified into five hierarchies with reference to the human's visual characteristic. In FIG. 2, the first hierarchy includes only DC components. The second, third and fourth hierarchies are form, in the same manner as that of doubling the number of the transform coefficients in the vertical (or horizontal) direction. The fifth hierarchy includes all transform coefficients contained in a macroblock. FIG. 3 shows an example of recording the video data of the I-picture on tracks in the divided manner according to the hierarchies described with respect to FIG. 2. In FIG. 3, portions indicated as black rectangles represent trac portions located on head tracks during sixteen-multiple, eight-multiple and four-multiple speed reproduction, and hatched rectangles represent track portions placed on the head tracks during the four-multiple speed reproduction. The other portions represent track portions located on the end tracks during the eight-multiple and the four multiple speed reproduction.
When performing a sixteen-multiple speed reproduction operation, since the video data of the I-picture exist on the head tracks for the sixteen-multiple speed reproduction indicated as a solid line in FIG. 3, the digital VCR can regenerate an effective image. The eight-multiple speed reproduction is the same case as the sixteen-multiple speed reproduction.
In case of a digital VCR using a magnetic tape on which the video data is recorded according to the above-described manner with reference to FIGS. 1 through 3, a head drum motor for running head and a capstan motor for transferring the magnetic tape are driven at a constant speed adapted for predeterminedly corresponding high speed reproduction. Accordingly, the running tracks of the head become linear. Thus, when the head drum motor and the capstan motor operate at different speeds from the predetermined speeds, the video data of the I-picture required for the corresponding high speed reproduction cannot be read from the magnetic tape because the I-picture video data is divisionally recorded on the various tracks. As a result, a time delay occurs when a servo portion controls the above speeds. Since the I-picture data necessary for the corresponding high speed reproduction is not restored, a sudden degradation of the picture plurality which is noticeable by the human's eyes takes place.